If you wish to unsubscribe from AdEspresso, you can go to your profile page: https://app.adespresso.com/billing/profile and click on the Cancel Subscription link available on Your Plan box.
Follow instructions and you are done. Your account will remain active with no restrictions until the end of the billing cycle, so you can keep working as usual until then.
After unsubscribing, your campaigns will remain available to use on Facebook, unless you delete them from AdEspresso (see here on how to do that). Please mind that if you delete your campaigns in AdEspresso they will be set as Deleted on Facebook too! (There is NO way back).
Hope to see you back soon!